Ophthalmic docking system with 3-dimensional automatic positioning using magnetic sensing array

ABSTRACT

A magnetic positioning system and related method for automated or assisted eye-docking in ophthalmic surgery. The system includes a magnetic field sensing system on a laser head and a magnet on a patient interface to be mounted on the patient&#39;s eye. The magnetic field sensing system includes four magnetic field sensors located on a horizontal plane for detecting the magnetic field of the magnet, where one pair of sensors are located along the X direction at equal distances from the optical axis of the laser head and another pair are located along the Y direction at equal distances from the optical axis. Based on relative magnitudes of the magnetic field detected by each pair of sensors, the magnetic field sensing system determines whether the patient interface is centered on the optical axis. The system controls the laser head to move toward the patient interface until the latter is centered on the optical axis.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to and is a continuation of U.S. patentapplication Ser. No. 15/786,484, filed Oct. 17, 2017, the entirety ofwhich is incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to docking of an instrument head to a patientinterface device during laser ophthalmic surgery, and in particular, itrelates to devices, system and method that aid automatic docking basedon automatic positioning using a magnetic sensing system on theinstrument head and the patient interface device.

Description of Related Art

Significant developments in laser technology have led to its applicationin the field of ophthalmic surgery, and laser surgery has become thetechnique of choice for ophthalmic surgical applications. Ophthalmicsurgery is a precision operation and requires precise coupling betweenthe surgical tool (i.e., the laser beam) and the region to be surgicallyaltered (i.e., a portion of the patient's eye). Movement of the eye withrespect to the intended focal point of the laser beam can lead tonon-optimal results and could even result in permanent damage to tissuewithin the eye. Given that eye movement is often the result of autonomicreflex, techniques have been developed in an attempt to stabilize theposition of a patient's eye with respect to an incident laser beam.

Mechanical stabilisation devices, referred to as patient interfaces(PI), have been developed for coupling the patient's eye to the lasersystem. A PI typically has a component that directly contacts the eye,and engages and stabilizes the eye; meanwhile, the PI is attached to thelaser system, so that the laser beam can be aligned to the eye.Currently available designs of PIs typically have either a single-pieceor a two-piece structure.

Using a two-piece structure, the surgeon installs a lens cone on thebeam delivery head of the laser system, and installs a suction ringassembly on the patient's eye using a suction force, and then docks thetwo pieces (lens cone and suction ring assembly) together using themotorized gantry of the laser system. In a single-piece structure, thelens cone and the suction ring assembly are integrated as one piece. Insome systems that use a single-piece PI, the surgeon first installs thePI on the patient's eye, and then brings the laser head to the vicinityof the PI using the motorized gantry, and docks the laser head with thePI. A single-piece PI, or the piece of a two-piece PI that contacts theeye, is typically a single-use item intended to be used only once.

SUMMARY

Embodiments of the present invention provide a magnetic positioningsystem and related method for automated or assisted eye-docking inophthalmic surgery. The system includes a magnetic field sensing systemprovided on the laser head and one or more magnets provided on the PI.

Advantages of embodiments of the present invention include: Automationor assistance of eye docking in the treatment workflow enhances thealignment accuracy and also shortens the treatment time. Both willimprove the diagnostic and treatment outcome. The shortened treatmenttime also contributes to patient comfort.

Additional features and advantages of the invention will be set forth inthe descriptions that follow and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims thereof as well as the appended drawings.

To achieve the above objects, the present invention provides anophthalmic surgical laser system, which includes: a laser delivery head,including optics which define an optical axis for delivering a laserbeam to an eye of a patient; an magnetic field sensing system, whichincludes a first, a second, a third and a fourth magnetic field sensorsand a control device electrically coupled to the first through fourthsensors, wherein the first through fourth sensors are affixed on thelaser delivery head and located in a plane perpendicular to the opticalaxis, wherein the first and second sensors have identical structures andare located at equal distances from the optical axis along a first linethat passes through the optical axis, wherein the third and fourthsensors have identical structures and are located at equal distancesfrom the optical axis along a second line that passes through theoptical axis, and wherein the control device is configured to controleach of the first through fourth sensors to measure a magnetic fieldgenerated by an external magnet, and based on the measured magneticfield signals by the first through fourth sensors, to determine whetheror not the external magnet is located within a predetermined distancefrom the optical axis.

In another aspect, the present invention provides a patient interfacedevice for use in ophthalmic surgery, which includes: a body having around shape and defining a central area for accommodating an opticalpath of a laser beam; an annular flexible skirt located at a lower endof the body; and a ring shaped magnet disposed on the body, the magnetbeing centered on a rotational axis of the body.

In yet another aspect, the present invention provides a method fordocking an ophthalmic surgical laser system to a patient's eye, thelaser system including a laser delivery head which defines an opticalaxis for delivering a laser beam into the patient's eye, a mechanicalstructure configured to move the laser delivery head, and a magneticfield sensing system, the magnetic field sensing system including firstto fourth magnetic field sensors and a control device, wherein the firstthrough fourth sensors are affixed on the laser delivery head andlocated in a plane perpendicular to the optical axis, the first andsecond sensors have identical structures and are located at equaldistances from the optical axis along a first line that passes throughthe optical axis, and the third and fourth sensors have identicalstructures and are located at equal distances from the optical axisalong a second line that passes through the optical axis, the methodincluding: (a) controlling each of the first through fourth sensors tomeasure a magnitude of a magnetic field generated by an external magnet;(b) based on the measured magnetic field magnitude by the first throughfourth sensors, determining whether or not the external magnet islocated within a predetermined distance from the optical axis; and (c)based on the measured magnetic field magnitude by the first throughfourth sensors, controlling the mechanical structure to move the laserdelivery head toward the external magnet.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an ophthalmic surgical laser systemincorporating a magnetic field sensing system for automatic or assistedeye docking according to an embodiment of the present invention.

FIG. 2 schematically illustrates the structure of the magnetic fieldsensing system according to an embodiment of the present invention.

FIG. 3 schematically illustrates a patient interface incorporating amagnet according to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Eye docking is a critical first step in many ophthalmic diagnostic andtreatment procedures. Currently all known eye docking systems requiremanual manipulation of the instrumentation head (e.g. laser head) inthree dimensions to align the instrumentation head to the PI piece thatis installed on the eye. The manipulation is typically performed usingjoystick or other input devices, either real or virtual, with the aid ofa video camera. The feedback to the surgeon is the image showing the eyeand parts of the laser head, which requires manual interpretation. Thisalignment requires dexterity and careful attention by the surgeon.Inexperience can significantly prolong the overall treatment time andadd to patient discomfort. In current procedures, the only registrationautomation occurs after the eye is docked in place, i.e., after thelaser head is coupled to the PI and the optical imaging system withinthe laser head is able to acquire images of the eye through the PI.

Embodiments of the present invention provides a magnetic positioningsystem and related methods that aid automatic docking in the “mid-range”of the overall eye registration operation, i.e., after the laser head isbrought to within, for example, approximately 1 foot (approximately 30cm) of the PI piece that has been installed on the eye, and before thelaser head is sufficiently aligned and close to the PI piece such thatthe optical imaging system within the laser head is able to acquireimages of the eye through the PI. Embodiments of the invention providefor automatic detection of relative position of the laser head withrespect to the PI piece when they are within the operating range ofapproximately 1-2 feet from each other; the relative positioninformation is used to automatically, without operator intervention,move the laser head toward the PI piece until the laser head is within asufficiently close distance to the PI piece in at least the transversedirections (directions perpendicular to the optical axis of the laserhead), e.g. within a few mm, e.g., 3 mm. Position detection isaccomplished using magnetic field sensing.

More specifically, as schematically illustrated in FIG. 1, the magneticpositioning system includes one or more magnets 20 provided on the PIpiece 200 that is mounted on the patient's eye, and a magnetic fieldsensing system 10 with multiple magnetic field sensors provided on thelaser head 100 to detect the magnetic field generated by the magnet. Themagnetic signals detected by the multiple magnetic field sensors areused to determine or estimate the 3D position of the magnet and to movethe laser head toward the PI piece. The laser head 100 includesmechanical structures controlled by a controller to move the laser headin X, Y and Z directions. Note that the illustration in FIG. 1 is highlyschematic and is not intended to represent the actual shape, size,proportion, or precise physical location of the various components.

Embodiments of the present invention are applicable to both a laserophthalmic surgery system that employs a two-piece PI, where the magnetis provided on the PI piece that is installed on the eye, and to a laserophthalmic surgery system that employs a single-piece PI, where themagnet is provided on the PI and the PI is installed on the eye beforeit is docked to the laser head. In the descriptions herein, forconvenience, the PI piece that has the magnet provided on it is referredto as “the PI” 200 for both types of systems.

Preferably, the one or more magnets on the PI have their north and southpoles oriented in the direction parallel to the central axis of the PI,which is expected to be oriented in the vertical (Z) direction duringthe eye docking operation.

The magnetic field sensing system located on the laser head includesmultiple magnetic field sensors, each sensor detecting the X, Y and Zcomponents (i.e. a vector) of the magnetic field at its location. Themagnetic field sensing system also includes a control device (which mayinclude hardware circuits, a processor with memory storing computerprograms, or other types of circuitry) for processing the signalsgenerated by the magnetic field sensors, as well as a power source. Anysuitable magnetic field sensors may be used. The multiple sensors arelocated at fixed locations on the laser delivery head. For example, thelaser head may have a bottom surface from which a cone shaped housing101 (see FIG. 1) protrudes; the sensors may be mounted on the bottomsurface around this housing. Based on the magnitude of the magneticfield detected by different sensors, the control device determines therelative location of the magnet with respect to the sensors, andaccordingly controls the movement of the laser delivery head to centerit relative to the PI for docking to the PI.

In one embodiment, schematically illustrated in FIG. 2, the magneticfield sensing system employs five magnetic field sensors A-E. Four ofthe sensors A, B, C and D are identical in structure and located in anX-Y plane (horizontal plane which is perpendicular to the optical axisof the laser head) at the same distance from a center position O in the−X, +X, −Y and +Y directions, respectively. The distance from the centermay be in the range of 1 to 100 cm, preferably 5 to 20 cm, morepreferably about 10 cm. As will be discussed later, other sensorconfigurations are possible. The fifth sensor E is located above orbelow the X-Y plane. For example, the fifth sensor E may be locatedwithin or on the housing 101 (see FIG. 1). The fifth sensor E isoptional. The center position O is the intersection of the X-Y plane andthe optical axis of the laser beam to be delivered to the eye, whichalso corresponds to the intended docking position of the PI; i.e., thePI is deemed to be correctly docked when the magnet is located at theposition (0, 0, −z) in the X-Y-Z coordinate system, Z being the verticaldirection and parallel to the optical axis of the laser head. FIG. 2schematically shows the magnet 20 being at an off-centered position.

When the magnet is located between the sensors A and B in the Xdirection, the magnitudes of the magnetic field detected by sensors Aand B are used by the control device to determine the position of themagnet along the X axis. Likewise, when the magnet is located betweenthe sensors C and D in the Y direction, the magnitudes of the magneticfield detected by sensors C and D are used by the control device todetermine the position of the magnet along the Y axis. The magnet isdeemed to be centered in the X-Y directions when the magnitudes of themagnetic field detected by the sensors A and B are equal, and themagnitudes of the magnetic field detected by the sensors C and D areequal. The magnitude of the magnetic field detected by the fifth sensorE relative to the average magnitude of the in-plane sensors A to D isused by the control circuitry to determine the relative position of themagnet along the Z axis. The relative magnitude detected by the sensor Ethat corresponds to the correctly docked position of the PI may beestablished empirically.

Here, those skilled in the art would appreciate that when two quantitiesare said to be equal, what is meant is that their difference is lessthan a threshold which may depend on noise level in the signal andinstrument limitations.

In addition to the magnetic field generated by the magnet on the PI,there exist naturally occurring background magnetic fields and magneticfield gradients, either static or time-varying. By using a plurality ofsensors in the manner described above, such background magnetic signalsare measured and compensated for.

As the goal of the magnetic positioning system is to accurately andprecisely center the magnet at the center position O defined by themultiple sensors of the magnetic field sensing system, it is in fact notcritical to precisely determine the position of the magnet when it is atan arbitrary off-centered position; what is important is to determinewhether the magnet is precisely centered. As pointed out above, whetherthe magnet is centered can be determined by whether the magnitudes ofthe magnetic field detected by sensors A and B, and by sensors C and D,are respectively equal to each other. By using the multiple sensors A-Din the configuration described above, the precision of the positioncentering within a few mm or better can be achieved. In other words, themagnetic field sensing system is able to determine whether the magnet islocated at within a few mm or less from the center position O defined bythe multiple sensors (i.e. from the optical axis of the laser head).

Thus, when the magnet is at a location relatively far away from thecenter position, mm level of precision is not required; it is sufficientfor the magnetic field sensing system to determine the approximateposition of the PI or the approximate direction (in the X-Y plane) thatthe laser head needs to be moved in order to move it toward the PI. Themagnetic positioning system is also able to estimate the approximateposition of the PI when the PI is located outside of the square regionbound by the four sensors A-D.

In some embodiments, the operating range of the magnetic positioningsystem, i.e. the farthest distance of the magnet from the centerposition O of the magnetic sensors such that the system can reliablyoperate to bring the laser head toward the PI, is approximately 1 foot(30 cm) or more. An operating range of 1 foot is sufficient for thepurpose of automated docking, i.e., the surgeon only needs to manuallymove the laser head to within 1 foot from the PI.

During automated eye docking, the magnetic positioning system controlsthe movement of the laser head via a laser head movement controller. Ina preferred embodiment, the laser head is initially position at a heightabove the PI, and controlled by the magnetic positioning system toautomatically move in the horizontal (X-Y) plane first to center itabove the PI, and then controlled to move in the vertical (Z) directionto lower it to dock with the PI. The magnetic positioning system maycontrol the movement of the laser head using various modes, includingcontinuous, stepwise, trial and error, etc., or combinations thereof. Ina continuous mode, the laser head is controlled to move continuously inone direction in the X-Y plane, and the magnetic field sensing systemcontinuously monitor the signals from the multiple sensors A-D toprovide feedback signals to maintain or change the movement speed and/ordirection. In a stepwise mode, the signals from the sensors A-D aremeasured and evaluated to estimate a horizontal direction of movementthat will bring the laser head closer to the PI; the laser head iscontrolled to move in that direction by a certain amount (withoutcontinuous monitoring of the magnetic signals); and the signals from thesensors A-D are measured again and evaluated to determine the next stepof movement. In a trial an error mode, which may be employed when thelaser head is located a relatively far away from the PI, the initialmovement may be in an arbitrary direction, and the magnetic signals aremeasured both before and after the initial movement to determine whetheror not the laser head has been moved in the correct direction. In all ofthese modes or their combination, the movement stops when the laser headreaches the center position as determined based on the magnetic signalsfrom the multiple sensors A-D.

After the laser head is centered above the eye, it is controlled to movedownwards to dock with the PI, as schematically shown by the dashed-linearrows in FIG. 1. As mentioned earlier, the magnitude of the magneticsignal detected by the fifth sensor E relative to the average signalmagnitude of the in-plane sensors A to D may be used to determine therelative position of the magnet along the Z axis, and control thedownward movement of the laser head. Also as mentioned earlier, thefifth sensor E is optional. When the fifth sensor is not used, the finaldocking movement in the Z direction may be manually controlled, orcontrolled by other feedback systems.

In addition to the sensor configuration shown in FIG. 2 and describedabove, other sensor configurations may be used. For example, in onealternative configuration, sensors A and B are at equal distance fromthe center O, and sensors C and D are at equal distance from the centerO, but the distance from sensor C (and D) to the center O is differentfrom the distance from sensor A (and B) to the center O. In anotheralternative configuration, the second pair of sensors C and D are notlocated along the Y axis, but are located along a line in the X-Y planethat passes through the center O but is at a non-orthogonal angle withrespect to the X axis. In yet another alternative configuration, sensorsA and B have identical structures and sensors C and D have identicalstructures, but sensors C and D have a different structure than sensorsA and B.

Some other alternative sensor configurations may use fewer or more thanfour in-plane sensors, such as three (e.g., forming an equal-sidedtriangle) or six (e.g., forming an equal-sided hexagon). Depending onthe number and locations of the multiple sensors, position determinationbased on the relative signal magnitudes may be more complex than thatusing the configuration of FIG. 2. One method for position determinationis to calibrate the system beforehand, by recording the relative signalmagnitudes among the sensors at multiple known PI locations (e.g. a gridof locations). A lookup table (LUT) may be constructed, and then used toestimate the location of the PI and to move the laser head toward thePI.

As shown in FIG. 3, the one or more magnet 20 may be provided on the PI200 at any suitable location or locations. The PI typically has a roundshape, such as a ring, a truncated cone, etc. The magnet is preferablylocate on the PI such that the shape of the magnetic field it generatesis centered on the rotational axis 201 of the PI. As mentioned earlier,the north and south poles of the magnets are preferably oriented in thedirection parallel to the rotational axis of the PI. Meanwhile, the oneor more magnets should be positioned without obscuring a central area202 of the PI that accommodates the optical path of the laser beam.Thus, one ring shaped magnet centered on the optical axis 201 may beused. Alternatively, multiple magnets may be used and distributed evenlyin a circle around the optical axis. For example, one or more magnets 20may be located on an upper rim 203 of the PI as shown in FIG. 3. FIG. 3also illustrates a flexible skirt 206 located at the lower end of the PI200, configured to contact the anterior surface of the patient's eyewhen the PI is mounted on the eye.

The inventors have constructed a test system using three magnetic fieldsensors and demonstrated that it can achieve position centering of themagnet within a few mm, with the centered position being about 8-10 cmfrom the sensors. The test system used a disc magnet having a diameterof about 10 mm and thickness of about 2 mm.

It is noted that the magnetic field vector in the area surrounding thePI is affected by both the translational position and the orientationangles (e.g. pitch and tilt) of the magnetic axis of the PI. In oneembodiment, the positioning method adopts a simplifying assumption thatthe surgeon holds the PI so that its N-S magnetic axis remainsapproximately aligned with the optical axis of the laser head. It hasbeen experimentally determined that in such a case, detecting only thescalar magnetic field strength is sufficient to accurately center the PIwithin a few mm. In this embodiment, while the sensors themselves arecapable of measure the X, Y and Z components of the magnetic fieldvector separately, the components are combined to form a scalarmagnitude which is used for positioning.

In the more general case that the PI, and hence its magnetic field, isallowed to significantly pitch or tilt away from the optical axis of thelaser head, the centering position accuracy will be less than a few mmunless the control device uses all three (X, Y, Z) components of themagnetic field separately at each sensor position. This additionalinformation enables the control device to estimate all degrees offreedom of the PI, e.g. the translational position in the X, Y, Zdirections simultaneously with the pitch and tilt angles. With a knowntotal magnetic field strength at the PI, that sensor data allows thevector magnitude of any external stray magnetic fields to be calculatedas well. That enables accurate alignment of the PI in five degrees offreedom, namely X, Y, Z, pitch, and tilt, even in the presence ofadditional stray magnetic fields.

In addition to eye docking in ophthalmic procedures, the automated orassisted docking system according to embodiments of the presentinvention may be useful in any surgical or diagnostic instrumentationthat requires alignment of the instrument to a specific body part, orother systems where one part is required to be physically aligned withanother part.

It will be apparent to those skilled in the art that variousmodification and variations can be made in the automatic docking systemusing magnetic positioning and related method of the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover modifications and variationsthat come within the scope of the appended claims and their equivalents.

What is claimed is:
 1. An ophthalmic surgical laser system comprising: alaser delivery head, including optics which define an optical axis fordelivering a laser beam to an eye of a patient; and a magnetic fieldsensing system, which includes three or more magnetic field sensors anda control device electrically coupled to the three or more sensors,wherein the three or more sensors are affixed on the laser delivery headand located in a plane perpendicular to the optical axis forming anequal-sided polygon centered at the optical axis, and wherein thecontrol device is configured to control each of the three or moresensors to measure a magnetic field, and based on the measured magneticfield signals by the three or more sensors, to determine whether or notan external magnet that has generated the magnetic field is locatedwithin a predetermined distance from the optical axis.
 2. The ophthalmicsurgical laser system of claim 1, wherein the predetermined distance is3 mm.
 3. The ophthalmic surgical laser system of claim 1, wherein thethree or more sensors have identical structures.
 4. The ophthalmicsurgical laser system of claim 1, wherein the magnetic field sensingsystem further includes an additional magnetic field sensor locatedoutside of the plane, wherein the control device is further configuredto control the additional sensor to measure the magnetic field, andwherein the control device is further configured to determine, based onthe measured magnetic field magnitude by the three or more sensors andthe additional sensor, a relative position of the external magnet thatgenerated the magnetic field in a direction along the optical axis. 5.The ophthalmic surgical laser system of claim 1, wherein the controldevice is configured to move the laser delivery head based on themeasured magnetic field magnitudes by the three or more sensors.
 6. Theophthalmic surgical laser system of claim 5, wherein the control deviceis configured to move the laser delivery head toward the externalmagnet.
 7. The ophthalmic surgical laser system of claim 1, wherein thecontrol device is configured to control each of the three or moresensors to measure a magnetic field vector generated by the externalmagnet, and based on the measured magnetic field vectors, to furtherdetermine a pitch and a tilt angle of the external magnet relative tothe optical axis.
 8. An ophthalmic surgical laser system comprising: apatient interface device which includes: a body having an annular shapeand defining a central area for accommodating an optical path of a laserbeam and a central axis within the central area; an annular skirtlocated at one end of the body; and a ring shaped magnet disposed on thebody along a perimeter of the annular shape and around the central area,the magnet being centered on the central axis of the body; a laserdelivery head, including optics which define an optical axis fordelivering a laser beam to an eye of a patient; and a magnetic fieldsensing system, which includes three or more magnetic field sensors anda control device electrically coupled to the three or more sensors,wherein the three or more sensors are affixed on the laser delivery headand located in a plane perpendicular to the optical axis forming anequal-sided polygon centered at the optical axis, and wherein the magnetof the patient interface device is configured to generate a magneticfield, and wherein the control device is configured to control each ofthe three or more sensors to measure the magnetic field, and based onthe measured magnetic field signals by the three or more sensors, todetermine whether or not the magnet of the patient interface device islocated within a predetermined distance from the optical axis.
 9. Amethod for docking an ophthalmic surgical laser system to a patient'seye, the laser system comprising a laser delivery head which defines anoptical axis for delivering a laser beam into the patient's eye, and amagnetic field sensing system, the magnetic field sensing systemincluding three or more magnetic field sensors and a control device,wherein the three or more sensors are affixed on the laser delivery headand located in a plane perpendicular to the optical axis forming anequal-sided polygon centered at the optical axis, and wherein thecontrol device is operatively coupled to the laser delivery head, themethod comprising: (a) installing a patient interface device on thepatient's eye, the patient interface device including a magnetconfigured to generate a magnetic field; (b) the control devicecontrolling each of the three or more sensors to measure a magnitude ofthe magnetic field generated by the magnet of the patient interfacedevice; (c) based on the measured magnetic field magnitude by the threeor more sensors, the control device determining whether or not themagnet of the patient interface device is located within a predetermineddistance from the optical axis; and (d) based on the measured magneticfield magnitude by the three or more sensors, the control device movingthe laser delivery head toward the magnet of the patient interfacedevice.
 10. The method of claim 9, wherein the predetermined distance is3 mm.
 11. The method of claim 9, wherein the magnetic field sensingsystem further includes a fifth an additional magnetic field sensorlocated outside of the plane, wherein the method further comprises: thecontrol device controlling the additional sensor to measure a magnitudeof the magnetic field generated by the magnet of the patient interfacedevice; and based on the measured magnetic field magnitude by the threeor more sensors and the additional sensor, the control devicedetermining a relative position of the magnet of the patient interfacedevice in a direction along the optical axis.
 12. The method of claim 9,wherein step (b) further includes the control device controlling each ofthe three or more sensors to measure three components of a magneticfield vector generated by the magnet of the patient interface device;wherein step (c) includes, based on the measured magnetic field vectorcomponents by the three or more sensors, the control device determiningwhether or not the magnet of the patient interface device is locatedwithin a predetermined distance from the optical axis; and wherein step(d) includes, based on the measured magnetic field vector components bythe three or more sensors, the control device moving the laser deliveryhead toward the magnet of the patient interface device.